The present invention relates to a smoke detector, and more particularly to a light-scattering-type smoke detector.
In a light-scattering-type smoke detector, when the smoke enters into its smoke-detecting chamber the light issued from a projecting element is adapted to be scattered and then be received by a light receiving element. Since the smoke-detecting chamber is formed so that it allows the free entry of ambient air, but not entry of outside light it is surrounded by a so-called labyrinth.
The labyrinth known hitherto was formed by a plurality of light shielding columns each having a T-shaped cross section and arranged in a circle, the columns being colored black and delustred. However, in conventional light-scattering-type smoke detectors, in order to improve the light shielding characteristics the heads of the T-shaped light shielding columns constituting the labyrinth were arranged to be in surfrace contact with the outer periphery of the labyrinth, the smoke entrance area formed at the outer periphery of the labyrinth then being very narrow. Therefore, in order to ensure the quantity of smoke necessary for detection the outer diameter of the labyrinth has to be made large, this necessarily making the size of the smoke detector large.
Further, since all of the light issuing from the projecting element to impinge upon the inner wall of the labyrinth is not absorbed there, the scattered light incidents upon the light receiving element so that the noise light output N of the light reflected from the inner wall of the labyrinth becomes large.
Therefore, the ratio of the signal light output S of the light scattered by the smoke to the noise light output N, i.e. S/N decreases, lowering the performance characteristics of this type of smoke detector.